Porphyroblasts and Reaction Rims

Abstract

Information contained in the structure in and around porphyroblasts and in reaction rims is discussed in this chapter. Porphyroblasts often contain inclusion patterns that reflect the structure of the matrix during their growth. Once formed, a rigid porphyroblast protects these patterns from further modification, whereas the matrix may be subjected to later crenulation, recrystallisation, grain growth etc. For this reason comparison of porphyroblasts and their surrounding matrix gives valuable information on the sequence of deformation phases and the relative time interval of porphyroblast growth with respect to these phases. A distinction is made between pretectonic, intertectonic, syntectonic and post-tectonic porphyroblasts. Using this technique a relative sequence of metamorphic events and deformation phases can be established.

Special inclusion patterns, such as millipede structures, deflection folds and oppositely concave microfolds are explained and discussed. Attention is also given to possible pitfalls in the interpretation of porphyroblast microstructures, such as inclusion shapes and patterns controlled by crystallographic directions rather then representing passive inclusions reflecting a pre-existing matrix foliation.

The relative rotation or non-rotation of porphyroblasts with respect to their surrounding matrix or to geographic coordinates is still a hot topic in the literature and a revision is given of the current state of the art.

A new topic refers to amalgamated porphyroblasts, a possible source of misinterpretation that may produce in some cases apparently enigmatic inclusion patterns.

The final part of this chapter deals with reaction rims, an important tool to unravel the metamorphic history of the rock. Attention is given to coronas, moats, symplectites and pseudomorphs.